Conventionally, a film having optical anisotropy is widely used as a retardation film, an optical compensation film, or a viewing angle widening film, etc., and has remarkably contributed to the improvement of the optical property of a liquid crystal display device. In this specification, all films, which can be used for various kinds of optical devices and have optical anisotropy, such as a retardation film, an optical compensation film, or a viewing angle widening film, are defined as “a retardation film”.
Regarding a retardation film, a variety of proposals have already been made in order to improve various kinds of required properties. Particularly, a technique for improving the viewing angle of a liquid crystal display device is important for a retardation film, and many proposals have been made for this purpose.
Patent documents 1 to 4, for example, describe the controlling of principal refractive indices in the three directions perpendicular to each other, and parallel or perpendicular to a plane of a retardation film (nx, ny and nz, referred to as three-dimensional refractive indices hereinafter). Specifically, Patent documents 1 to 4 describe that the principal refractive index in the thickness direction (nz) is larger than one of the two in-plane principal refractive indices (nx, ny) and is smaller than the other one. When the principal refractive indices in the three directions (nx, ny, nz) are controlled as described above, the viewing angle dependency of retardation of a retardation film can be controlled, and as a result, the viewing angle of a liquid crystal display device can be widened.
However, the methods described in Patent documents 1 to 4 utilize birefringence due to the orientation of a polymer, which constitutes a retardation film (referred to as molecular-orientation birefringence hereinafter), thus the properties of the resulting retardation film are limited. Also, in order to make the principal refractive index in the thickness direction (nz) have an intermediate value between the two in-plane principal refractive indices (nx, ny), a very complex stretching method has to be adopted. Also, because a very complex stretching method is applied in order to obtain a retardation film having such principal refractive indices, there are problems that the precise control of the retardation value is difficult and also the wavelength dependency of retardation cannot be sufficiently controlled.
As another method for achieving the wide viewing angle of a liquid crystal display device, a method of adhering plural retardation films with an adhesive, etc., to achieve the desired effect has been proposed. Patent document 5, for example, describes a technique for improving the viewing angle dependency of a retardation film by laminating a positive uniaxial optical film having an optical axis within a plane and a negative uniaxial optical film having an optical axis within a plane. According to the method of Patent document 5, the retardation can be controlled without adopting a complex stretching method.
However, the method described in Patent document 5 is a method of utilizing only molecular-orientation birefringence, thus the properties of the resulting retardation film are limited. Also, because the obtained optical properties are the result of a mixture of optical properties, the positive uniaxial optical film and the negative uniaxial optical film, it is difficult to freely control the optical properties, especially the wavelength dispersion of retardation.
Furthermore, as another method for achieving the wide viewing angle of a liquid crystal display device, Patent document 6 describes a method for exhibiting birefringence due to retardation between the in-plane direction and the thickness direction (referred to as form birefringence, hereinafter), wherein the method includes alternately laminating two groups of isotropic layers, which are made of inorganic materials having different reflective indices and do not express retardation as a single layer, to form a periodic structure. The laminated retardation film in Patent document 6 is intended to apply a negative C plate to a liquid crystal display device by utilizing form birefringence (the negative C plate is a plate having the same two in-plane principal refractive indices (nx, ny) and the refractive index in the normal direction to the surface (nz) smaller than the two in-plane principal refractive indices (nx, ny)). Patent document 6 describes an example wherein the viewing angle of a liquid crystal display device is improved by applying the film to a twist nematic (TN) liquid crystal display device.
However, the method described in Patent document 6 is a method utilizing only form birefringence, thus the properties of the resulting retardation film are limited. Also, in the method described in Patent document 6, only a multilayer structure having negative uniaxial anisotropy can be obtained.
As for a retardation film used in the field of optics, further improvement of the properties and more advanced controllability of retardation are strongly required, which continues to this day.
[Patent document 1] Japanese Unexamined Patent Publication No. 02-160204
[Patent document 2] Japanese Unexamined Patent Publication No. 04-127103
[Patent document 3] Japanese Unexamined Patent Publication No. 05-157911
[Patent document 4] Japanese Unexamined Patent Publication No. 07-230007
[Patent document 5] Japanese Unexamined Patent Publication No. 03-024502
[Patent document 6] U.S. Pat. No. 5,196,953